1. Field of the Invention
The present invention relates to a process for preparing butene oligomers, which are valuable starting materials for plasticizer alcohols, from field butanes. The process is highly flexible because the oligomer products may be determined by judicious selection of the starting materials.
2. Description of the Background
Dibutenes are an isomeric mixture which are formed, in addition to higher butene oligomers, by dimerization and/or codimerization of butenes, i.e., of n-butene and/or isobutene, in the oligomerization of butenes. The term di-n-butene refers to the dimerization product of n-butene, i.e., dimerization 1-butene and/or 2-butene. Important components of di-n-butene are 3-methyl-2-heptene, 3,4-dimethyl-2-hexene and, to a lesser extent n-octenes. The term dibutene may also refer to the dimerization products obtained by the reaction of n-butene and isobutene. Di-isobutene is the mixture of dimers which is formed by dimerization of isobutene. Di-isobutene contains molecules which are more highly branched than dibutene, and this, in turn, is more highly branched than di-n-butene.
Dibutene, di-n-butene and di-isobutene are starting materials for preparing isomeric nonanols by hydroformylation and hydrogenation of the C.sub.9 aldehydes thus formed. Esters of these nonanols, in particular the phthalic esters, are plasticizers which are prepared to a significant extent, and are primarily used for poly(vinyl chloride). Nonanols from di-n-butene are linear to a greater extent than nonanols from dibutene, which are in turn less branched than nonanols from di-isobutene. Esters of nonanols from di-n-butene, because of their more linear structure, have application advantages as compared to esters of nonanols obtained from dibutene or di-isobutene, and are particularly in demand.
Butenes can be obtained for the dimerization reaction to form dibutenes from the C.sub.4 fraction of steam crackers or of FC crackers, for example. This C.sub.4 fraction is generally worked up by first separating off 1,3-butadiene by a selective scrubbing, e.g., using n-methylpyrrolidone. Isobutene is a desirable and particularly valuable C.sub.4 fraction component because it may be chemically reacted to give sought-after products, e.g., with isobutane to provide high-octane iso-octane or with methanol to afford methyl tert-butyl ether (MTBE), which, as an additive to motor gasoline, improves its octane rating. After the reaction of the isobutene, the n-butenes, n-butane and isobutane remain behind. The proportion of n-butenes in the cracking products of the steam cracker or the FC cracker is relatively low, however, approximately 10 percent by weight, based on the principal target product ethylene. A steam cracker having the respectable capacity of 600,000 metric t/year of ethylene therefore only provides about 60,000 metric t/year of n-butene. Although this amount (and that of the isobutenes) could be increased by dehydrogenating the approximately 15,000 metric t/year of n-butane and isobutane which arise in addition to the n-butenes, this is not advisable, because dehydrogenation plants require high capital expenditure and are not economic for such a small capacity.
Isobutene is, as discussed above, a cracking product in high demand, and is therefore not generally available for the oligomerization reaction to produce n-butenes. The amount of n-butenes which a steam cracker or an FC cracker produces directly is not sufficient, however, to produce sufficient dibutene for a nonanol plant whose capacity is high enough that it could compete economically with the existing large-scale plants for preparing important plasticizer alcohols, such as 2-ethylhexanol. Butenes from several different steam crackers or FC crackers would therefore have to be collected and oligomerized together, in order to meet the dibutene demand of a large nonanol plant. Opposing this, however, is the fact that the transport of liquefied gases is expensive, not least because of the complex safety precautions required to do so.
It would be desirable if butenes could be provided at only one site without transport over relatively large distances in amounts for the oligomerization as are required for the operation of a large scale plant for preparing nonanols having a capacity of 200,000 to 800,000 metric t/year, for example. It would also be desirable to have a process for preparing butene oligomers in which the valuable di-n-butene product can be separated off from the dibutene product mixture. Finally, it would be desirable if the process could be controlled in such a manner that, in addition to higher butene oligomers, only di-n-butene or di-isobutene is formed as the dibutene product.